I see binutils and gcc support CTARGET in the APKBUILD files. I
naively tried:

cd main/binutils
CTARGET=armhf abuild -r

Which fails with:

build-base-armhf (missing)

I tried a few more things (for example faking a build-base-armhf), but
nothing worked. Is it possible to build an armhf compiler on x86_64
using abuild? How? At least abuild -h suggests that it is possible.